With Java 7 we have the problem that many JApplets , JDialogs and JFrams are empty. 
There is only a gray surface. After debugging and find the cause of the problem. 
Adding a component to the content pane does not invalidate the JApplet, JDialog or JFrame. 

In Java 6 and before the parent object was invalidate in this case.

 

Workaround:

Calling manual invalidate() after any change on the root objects.


Sample Code:

import java.awt.BorderLayout;
import java.awt.Container; 
import javax.swing.*;

public class TestApplet extends JApplet {
    @Override
    public void init() {
        validate();      

        JSplitPane split = new JSplitPane();
        split.setLeftComponent( new JLabel( "left" ) );
        split.setRightComponent( new JLabel( "Right" ) );
        getContentPane().add( split, BorderLayout.CENTER ); 

        Container container = split;
        while( container != null ) {
            System.err.println( container.getClass().getName() + " " + (container.isValid() ? "" : "invalid") );
            container = container.getParent();
        } 

        //workaround: call manual invalidate()
        //invalidate();
    }
}
 

Output from Java 6:
===============
javax.swing.JSplitPane invalid
javax.swing.JPanel invalid
javax.swing.JLayeredPane invalid
javax.swing.JRootPane invalid
TestApplet invalid
sun.applet.AppletViewerPanel invalid
sun.applet.AppletViewer invalid


Output from Java 7:
===================
javax.swing.JSplitPane invalid
javax.swing.JPanel invalid
javax.swing.JLayeredPane invalid
javax.swing.JRootPane invalid
TestApplet
sun.applet.AppletViewerPanel
sun.applet.AppletViewer